Zone electrophoresis in capillaries is widely used to accomplish liquid-phase separations of various solutes. Capillary electrophoresis has been used for separation of small and large molecules, various amino acids, alkylamines and various proteins. In brief, a capillary zone electrophoresis device includes a buffer-filled capillary tube that is placed between two buffer reservoirs. A potential field is applied across the length of the capillary tube, and then ionic solutes in one buffer reservoir differentially migrate through the capillary into the other reservoir. Small diameter silica based tubes are employed as the capillaries in capillary zone electrophoresis (CZE) instruments.
A distinguishing property of the capillary electrophoresis instrument is electroosmotic flow. Immediately adjacent to the solid-liquid interface at the interior of the silica-based capillary wall, a stagnant double layer of solute/solvent is found. Under normal aqueous conditions, the silica capillary wall surface has an excess of charge resulting from an ionization of surface functional groups. Thus, SiOH groups are ionized leaving SiO-- at the wall surface and H+ ions in the solution and in the stagnant double layer adjacent to the capillary wall. This action creates a potential across these layers, part of which is termed the zeta potential. The zeta potential is dependent upon the viscosity of the fluid, the dielectric constant of the solution and the charge on the inner surface of the wall of the capillary. The cationic counter ions (H.sub.3 O+, Na+ typically) in the diffuse solvent/solute layer migrate towards the cathode and because these ions are solvated, they drag solvent with them. The extent of the potential drop across the double layer governs the rate of flow. It is known that control of electroosmotic flow is effective in improving electrophoretic resolution and efficiency and is a controlling factor in obtaining reproducible results in a CZE apparatus.